Electrophotographic technology is widely used in a copying machine, a printer, and a printing machine since immediacy and high quality images can be obtained. Regarding electrophotographic photoreceptors (hereinafter, appropriately referred to as simply “photoreceptor”), which are the core of electrophotography technology, photoreceptors employing an organic photoconductive substance having advantages such as non-pollution, ease of film formation, and ease of production has been widely used in recent years.
In a case where the number of guaranteed sheets of an image forming apparatus is large, high repeat durability is also required for the photoreceptor. In order not to change the image quality over a long period of time, there is a need to reduce abrasive properties of the photosensitive layer and to prevent accumulation of surface adhered substances. In a case where a curing protective layer is provided, the abrasion resistance is improved, while no reface due to abrasion on the surface is made, so that adhered substances such as corona products, developers, and paper dust cannot be completely cleaned and tend to remain and accumulate. In the case where a curing protective layer is provided, dedicated production equipment is required, deterioration in coating liquid (insufficient storage stability) and deterioration in electrical properties also occur due to the functional group contributing to curing, and actually it is difficult to use it for ones other than high-end models.
In order to improve the durability without providing a curing protective layer, it is general practice to increase the abrasion resistance of the outermost charge transport layer. However, from the viewpoint that the abrasion resistance is not necessarily required for the entire film thickness of the charge transport layer, when the charge transport layer is formed as a plurality of layers, it is considered that electrical properties and adhesiveness other than abrasion resistance are regarded as important for the charge transport layer on the side close to the support, and abrasion resistance is applied predominantly for the charge transport layer of an outermost layer. Generally, a binder resin which constitutes a charge transport layer and has excellent abrasion resistance often has inferior electrical properties and adhesiveness, so that many ideas that such functional separation is effective are disclosed many times before.
As a proposal to improve abrasion resistance by a plurality of charge transport layers, Patent Literature 1 discloses an electrophotographic photoreceptor containing inorganic particles only in a first charge transport layer which is an outermost layer. In addition, Patent Literature 2 discloses an example in which a high molecular weight binder resin is used only for a first charge transport layer which is an outermost layer. Patent Literature 3 discloses a technique of increasing the hardness and elastic deformation ratio of a first charge transport layer which is an outermost layer. Patent Literature 4 discloses a technique of using a polyester resin having a specific structural unit in a first charge transport layer which is an outermost layer. Patent Literature 5 discloses a technique of forming layers in which a first charge transport layer as an outermost layer is excellent in scratch resistance and a second charge transport layer in contact with the first charge transport layer is excellent in potential stability and gas resistance, by using a copolymer resin where a plurality of charge transport layers have different units from each other and common units. In addition, unlike Patent Literatures 1 to 5, Patent Literature 6 discloses a technique for suppressing long-term image quality deterioration by using a binder resin having a higher molecular weight in a second charge transport layer in contact with a first charge transport layer which is an outermost layer and by increasing the film thickness of both end portions which are easy to abrade.